THIS invention relates to the multi-dimensional electronic identification of articles.
In radio frequency identification systems, a reader is typically used to read the identity of a number of objects in the form of transponders which are attached to goods to be identified, and to communicate the identification information to displays or computer networks. Communication between the reader and the transponders is by electromagnetic means, allowing transponders to be identified that are not in line of sight with the reader. Invariably, the transponders are randomly oriented, with the result that a single reader with a single energy polarisation is not capable of detecting all of the transponders. Nulls or dead zones also occur in the radiation pattern of a single reader due to interference patterns from reflecting surfaces and the like.
The problems associated with differences in polarisation and nulls or dead zones generally arise in the case of passive transponder systems, where the transponders are powered by an energising field, typically for the time period that such energising field exists. This is even more prevalent in the case of systems where transponders are effectively deactivated once they have been successfully identified, so as not to interfere with the transmissions of other transponders in the reading volume. Transponder systems of this type are described in U.S. Pat. No. 5,751,570 to Stobbe et al, U.S. Pat. No. 5,124,699 to Tervoet et al. and South African patent 93/6267 to Marsh et al.
In U.S. Pat. No. 5,519,381 to Marsh et al, a system is described that simultaneously uses at least two different interrogation signals at different frequencies to provide power to transponders in an interrogation volume. The different frequencies may be radiated from different angles, thereby compensating for dead spots and polarisation. U.S. Pat. No. 5,850,181 to Heinrich et al describes a system providing power transferred to a number of transponders by pulsing the electromagnetic field at randomly selected frequencies. The electromagnetic field is briefly turned off between frequency changes to limit spectral noise caused by the switching process. The transponders make use of an energy store to maintain the level of the operating voltage of the transponders whilst the frequency is switched between pulses.
In order to provide for low production costs, the electronic components of transponders need to be built into a single integrated circuit. As a result, the transponders have limited on-board energy storage capacity in situations where the energising field has been removed. In the absence of an on-board power supply in the form of a storage capacitor, such transponders are volatile, in that they tend to perform a power on reset when the energising field is re-established at the start of the next reading session, even when this occurs relatively soon after a prior reading session. This can lead to a single transponder being read several times in error in the case of multiple interrogation fields being utilized.